NONLINEAR GAIN FOR A SUBNOISE-LEVEL SIGNAL IN A DIGITAL SPECTRUM ANALYZER

In order to observe a subnoise-level signal, that is, a low-level sign al with a comparable or smaller amplitude of intruding noise: an avera ging over a multiple number of samplings is inevitable. Under these co nditions, if an analog-to-digital converter (ADC) is used, the respons e of the converter deviates from a step function. For a signal range o f about 50 dB, we compare the measured response of the spectrum analyz er with the simulated response of an ADC for a Gaussian noise distribu tion. This effect explains the disagreement between the expected and m easured value of the blackbody radiation in a cavity reported in Jpn, J, Appl, Phys. 36 (1997) 2399.